In hot rolling of steel products, in recent years, successive joining of continuous steel products for hot rolling followed by continuous rolling at a predetermined rate, that is, the so called "continuous hot rolling method," has been attempted in order to minimize a deterioration in yield of products on order caused by the creation of scratches or an unacceptable irregular shape due to a strainless state in the so-called "nonstationary portion" particularly in the front end (front portion) and the tail end (tail portion), unacceptable sheet width and thickness due to sledding speed, unacceptable temperature and unacceptable surface quality due to acceleration and the like and, in addition, to minimize removal of defect portions and sheet or plate travelling for finishing operation.
In this continuous hot rolling method, the front end and the tail end of a roughly hot rolled steel product to be fed into a continuous hot rolling mill, or a continuous steel product for hot rolling, such as a high-temperature, small-thickness continuous cast strip (in a sheet or coil form) are previously cut by means of a flying crop shear, the whole area or part of the cut face of the tail end in one steel product is welded to the cut face of the front end in another steel product, a number of continuous steel products for hot rolling are successively hot rolled according to an identical rolling schedule, or alternatively while continuously altering in a relay manner a plurality of schedules, and the hot rolled steel product is then cut and divided and coiled alternately using a plurality of coilers.
Butt electric welding, pressure joining, laser welding and the like have been studied as the welding means. Among them, laser welding is advantageous from the viewpoints of rapidity, joint strength, and quality of the joint.
A laser welding method is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 4-237584. In the method disclosed in this publication, when end faces of a high carbon steel cut by a laser are abutted against each other followed by laser welding, the laser welding is performed while feeding a filler wire containing 0.3 to 10% by weight of aluminum to the joint.
Specifically, this method is characterized in that laser cutting of a high carbon steel sheet containing not less than 0.5% of carbon permits oxygen evolved from an oxide film formed in the cut face to be fixed as an oxide, enabling joining to be performed even in the step of heating in welding at a very high temperature without creating weld defects from the oxide derived from foaming of the oxide, such as blow holes.
The above publication, however, does not disclose any method for continuously joining a high-temperature steel material or strip of 900.degree. C. or above.
In joining between the cut face of the tail end of one high-temperature steel product and the cut face of the front end of another steel product by laser welding, the amount of scale produced in the cut face of the end portions of the steel products is an important issue.
Specifically, the amount of the scale varies greatly upon the stand-by time defined by the time taken from the completion of cutting of the end portion of the steel product to welding (hereinafter referred to as "atmospheric exposure time") and the chemical composition of the steel product.
In general, scale constituted by an oxide formed on the surface of the steel product and air present around a molten joint are incorporated into a melt pool formed by laser beam irradiation and react with carbon in the steel product to evolve a CO gas. This tendency is more significant in hot joining than in cold joining. The CO gas is evolved not only from the surface of the melt area but also from the interior of the melt, resulting in foaming, which is left as a blow hole. This deteriorates the joint strength between the steel products, leading to a possibility that the joint between the steel products is broken during rolling.
For this reason, the amount of the scale formed in the end portion of the high-temperature steel product should be accurately detected to accurately remove oxygen, evolved from the scale, based on the amount of the scale.